Wave filter



July 19, 1927. L E SHEA WAVE FILTER Filed Aug. 2o; 1925 Patented July 19, 1927.

1,636,152` PATENT OFFICE.

TIMOTHY E. SHEA, 0F RUTHERFORD, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION 0F NEW YORK.

WAVE rin'rnn.

This invention relatesto wave lters and more particularly to methods of terminating wave iilterswhereby advantageous transmis sion characteristics may be secured.

In the design of broad baud wave filter-s it has heretofore been the common practice to proportion the wave iilter in accordance with the impedance of thelines or apparatus to which it is connected' so as to secure the most uniform transmission over ,the whole transmitting range. y

In many applications of wave lters to selective `transmissionI systems, it lis not necessary that transmission within the band be uniform at all'frequencies, and Vin such cases it has been found that advantages may be secured both in respect of economy of materials and of selectivity of transmission by suitably proportionin the relat-iveinipedances of the filter an its connected apparatus.

An important application ofthe inven-v tion, is in the construction of wave filters for connectingr carrier wave signalling apparatus to high voltage power lines, the advantage gained being a great reduction in the capacity of the high voltage coupling condensers and a corresponding reduction in their cost.

A general object o f the invention is to control the characteristics of frequency selective systems by proportioning the impedance of wave iilters by means of which the frequency selectivity is secured.

Another object is to reduce the cost of coupling circuits for supplying high. fre' quency signalling waves to high voltage power lines.

Another object is to improve the transmission characteristics of coupling clrcuits 1n duplex, or two-way, power line. carrier telephone systems.

A feature of the invention lies in the proportioning of wavefilters to have impedances greatly different from the impedances of the apparatus to which they are connected.

In theidetailed description that follows.

certain theorems of wave ltransmission will be discussed by means of which the nature of the invention may be more clearly under1 stood. One application of the invention, namely, to power line carrier wave signalling systems will also be described. It is to be understood,- however, that the invention is not limited to this particular application,

but only by the scope ofthe subjoined claims.

Of the accompanying drawings, Fig. l, represents in schematic. form a'transmission 'system .in which the invention is embodied; Figs. 2, 3 and 4 are theoretical `4diagrams for explanation purposes; and Fig. 5 illustrates the nature ofthe modification of the transmission characteristic obtained by the lapplication of the invention. The system of Fig. 2 represents 'in sche* matic form abroad class of wave transmission systems, in whichtwo terminal networks are linked together through a passive linear transducer, a wave source being included in one of the terminal impedances.

-A passive linear transducer is defined as an electrical network having` two input ternected between a sending end impedance ZB,

in which is generated an E. M. F. Ea, and a receiving end impedance Zh. In the` general case the structure within the dotted lines is restricted only to conform to the definition given above. It maybe,

for example, along transmission line, or' a wave filter, or it may comprise any arbitrary combination of invariable elements. The terminal impedances may likewise represent any kind of wave generating or wave receiving apparatus.

The input and the output currents, denoted respectively by I, and Ib, corresponding1 to the E. M. F. Ea, may be computedin terms of the terminal impedances and the impedances of the individual elements constituting the transducer, but it is more convenient and more illuminating to determine these currents from a set of'much more general properties of the transducer to which the name image parameters has been given. l

The image parameters 'comprise the image impedances, the transfer constant, and the directional transfer constants. The image impedances are two in number, corresponding to the two pairs. of terminals. In the. analysis that follows they are denoted by W,l and Wb for the input and output terminals res ectively.v They are defined as the impe ances by which the transducer may be terminated withoutt-he introduction of reflectio-n loss. In other words if the terminal impedances Za and Zh are equal to the ima e impe-dances Wn and Wb respectively, then t e Athe transducerA when the terminal impedan-ces are equal to the image impedances. Expressed mathematically afloat@ 1 when transmitting from termina-ls a to terminals b,.and

Tb. log, (2)

when transmitting from terminals b to terminals a, the currents in the second case being distinguished from those of the first case by prime marks. The limitation as to the terminal impedance is implied in each case.

The remaining parameter, the transfer The first factor. in equation .5 givesthe current that would flow Vif the two terminal impedances were connected together through an ideal impedance matching transformer. It is useful to consider this as a standard reference current with respect to which the gain or loss due to the inserted apparatus may be measured. The second and third factors give the separate reflection losses at the junctions to the terminal impedances. The factor c is the transfer factor, and the divisor (1-7,rbc2T), which has been called the interaction factor, represents the summation effect of repeated reflections from one end of the system to the other.

If the reflection coefficients 1'.. and rb are both made equal to Zero, as may be done by 100 making Z, equal to W8L and Zb equal to Wb,

the third and'fourth factors and the interacwww.

constant T, is equal to the arithmetic mean of Tw and Tb., or

It is important `to note that the quantities thus defined are parameters of the transducer itself without regard to the impedances to which the transducer may be connected. The serve therefore ,to describe the properties o the transducer in broad terms.

For a complete analysis of the transmis@` sion through a transducer system and for a description of important relationships between the imageparameters and other coefficients of a network, reference is made to the technical paper entitled Transmission Characteristics of Electric Wave Filters by Otto J. Zobel, published in the Bell System Technical Journal, Vol. III, No. 4, October,

-1924, and particularly to'AppendicesI 'and II thereof.

The following equation, given yby Zobel, expresses the receivedcurrent Ib in terms of the image parameters.

I d 2En/W,Wbe-T bi* (Wx'iZa) (l Turkse-ZT) .Vherc i, and rb, the current reflection coefficients at terminals a: and b, are respectively and zW.,.-Z., fb Equation 4 maybe rewritten in the' followinfr form in which the component losses are effectively displayed.

tion factor become unity and thcequation' reduces to If the matching of the inipedances is performed at one end only, the interaction factor still reduces to unity, but only one reflection loss factor, namely that corresponding to the matched end of the system, will reduce to unity.

Matching the impedances at both ends has the effect of eliminating all reflection losses, including that part which may be attributed to the inequality of the terminal impedances, and leaving only vthe inherent loss of the ,transducer as expressed by its transfer constant.

Since the definition of the transfer conequality over stant, `as the mean of the two directional transfer constants, may not readily convey an idea of its physical nature, i-t may be stated that the transfer constant determines the energy, or volt-ampere, attenuation in in-which V., and Vv, are the voltages at ter-4 minals a and respectively. This relation ship follows readily from equations 7l and f2.given in the above nientioned'paper.

ln most practical cases the image impedances are not constant quantities but vary with frequency in such manner that it is diicult, if not impossible, to construct ter-k ininal apparatus having impedances to `match the image impedaiices at more than a few` selected frequencies. The common practice is .tosecure a good approximation to the important frequency ranges and the degree of equality secured is reflected in the closeness with which the transmission characteristic of the Whole `system corresponds to that of the transducer alone as defined by equation 6.

The image parameters are related by simple formulae Vto the open circuit and the short circuit impedances of the transducer. The derivation of the relationships is given l in the article by Zobel already referred to,

and the results are therefore given here Without the mathematical development.

Let X, and Ya be the impedances of the ,transducer i'reasured at terminals a with terminals?) Qpen-circuited and short-circuited respectively, and let Xb and Yb be the corresponding impedances for terminals b,then

With the help of these formulae the image parameters may be determined from measurements of impedance made on the transducer alone, or by computation from the coetlicients of its structural elements.

Electricwave filters vconstitute a special class of transducers characterized'by being composed of a chain of networks each of which includes only reactive impedance elements. In practice, of course, it is not possible to constrict impedance elements en tirely free of resistance, but it has been found as the result `of much experience that the properties of a purely reactive system are very closely approximated so long as the resistances of the elements do not exceed one fiftieth of their reactances.

The weltki/iown property of Wave filters is their ability to transmit wave energy freely withwin certaiiidefinite bands of frequencies and toattenuate the energy atiall other frequencies. y

The limits of the transmission bands may be vdetermined by an'cxamination of the image impedances of the filter.` '.Since the filter is assumed to be composed of purely reactive elements it follows that the open circuit and short circuit iin'pedances must also be pure reactances. The image impede ances W.. and lVf, however are not necessarily pure reactances but may be either pure vreactances or pure resistances. Wa isa pure reactance if X,L and Ya are reactances of the saine sign, that is if both are inductive together or capacitive together. On the other hand if Xa and Ya are of opposite sign W a is a pure resistance. This follows from a consideration of equation 8. In addition, since Ya Y it follows that Wa and Wb are both reactive together and resistive together.

If now the `filter is assumed'to be tei-ininated in its image impedances, then for certain frequency ranges the Whole system including the terminals is reactive and at certain ,other frequency ranges itis resistive. In the first case, there can be no transmission of energy, but only a surging back and forth of energy from the input to the output of the system; whereas, in Athe second case, energy may be freely transmitted through the system and absorbed in the receiving impedance. i y

The transmission bands therefore coincide with the frequency ranges` for which the image impedances are pure resistance. If' a transducer is symmetrical in its schematic form With respect to thedi'rection of transmission, the two image impedances have the saine value. Similar symmetrical. transducers can therefore be connected in tandcni without introducing reflection losses -at the junction points, and the image impedanccs of the whole chain Will be the saine as those of the individual networks.

Uniform wave lters are constituted by such `achain of symmetrical sections, the common schematic forms of the sections being the well-known T, and 1r networks. T he T sections are sometimes called mid-series sections, and the yr sections, mid-shunt, in

,view of the process of division whereby they nia-y be derived from an intended uniform wave filtrer. The image impedances of the symmetrical sections are equal to their midian section iterative impedances, and the traiisfer constant is equal to the propagation constant, in terms of which the characteristics of the filter are commonly defined.

For the 'purpose of obtaining dierent typesof terminationA it is common practice to include a half section in a wave filternetwork, the half of a symmetrical T section' being identical with the half of a corresponding symmetrical 1r section.

In Fig. 2, for example, the transducer 'is part of a uniform wave filter having series branches and shunt Abranches of impedances Z1 andZ2 respectively, and comprises one symmetrical fr section and one half section.

The half section, being unsymmetrical, has

two image impedances, one equal to thatof the symmetrical vr section and one equal to that of the corresponding symmetrical T section.

The two image impedances of the system may be computed in terms of Z1 and Z2 with the help of equationsS, and when this is done it is -found that their ratio is given .by .l

lli.. i Wfl-Fizz (10) system at certain definite frequencies.

In-accordance with the present invention this principle is employed-in the design and construction of transmission systems to vsecure advantageous results.

In Fig. 1 is shown Vin simplified schematic one terminal of a power line carrier telephone system in which the invention finds application. The principal units of this system are a speech frequency line 1, 1, a twoway carrier translatingr system 2, a coupling filter 3, and a high voltage power line 4, 4.

Only the essential elements'of the system are shown, the detail arrangements of the circuits being capable of many variations, one of which is disclosed in the co-pending application of lVallace V. Wolfe, Serial No. 664147 filed September 22, 1923.

The carrier translating system 2 comprises z-tbridge coil 5 and line-,balance 6. whereby y directional separation of the speech currents is secured. Speech currents outgoing to the line 4, 4 enter the modulator-amplifier 7 wherein they are caused to modulate a high frequency carrier wave in any well known manner. The modulated high frequency wave is selected by a band pass wave filter 8 and transmitted to the terminals 9 of cou-V pling filter 3, through which it is transmitted to the power line 4.

Waves incoming from4 the power line also ypass through-the coupling filter and on reaching terminals 9 are diverted to the receiving channel of the ltranslating system by a band filter 10 of appropriate range. From filter 10 the incoming modulated waves pass todetectorll in which the speech waves are reproduced a-nd finally transmitted to the telephone line 1 through bridge coil 5.

To permit separation in the translating system of the outgoing and they incomin carrier waves they must necessarily have di ferent frequencies, and the transmission range of couplin filter 3 must be. great enough to permit lboth waves to pass freely. In` one practicalembodiment it has been found satisfactory to use vcarrier frequencies of 75 K. C. and 105 K. C. for the two waves and to provide for the transmission in the coupling filter bf a band of frequencies extending from 70 K. C; to 110 K. C.

The coupling filter 3 is. of `the type described in U. S. patent to G. A. ACam bell No. 1,227,113v issued May 22, 1917. ach series branch consists of a simple resonant circuit and each shunt branch consisting of a simple anti-resonant circuit, the frequencies 'or resonance and anti-resonance 4being equal. At the end next to the power line the filter terminates in series branches in which are included high-voltage condensers 12 designed to withstand the high voltage of the power line. -At the end next to the translating apparatus the filter terminates in a shunt branch.

The complete structure includes one 1r section andone half section, corresponding to the arrangement shown in Fig. 2, but additional sections may be added as desired.

The image impedances of the filter are plotted in Figs. 3 and 4 as functions of the frequency. Fig. 3 refers to the power line end of the filter and Fig. 4 to the opposite end. The portions of the impedance curves shown dotted represent reactances and the band limits are indicated by f, and f2 at which points the impedances change from reactance to resistance.

Within the band image impedance of the T section, or of the mid-series terminated end, is zero at each band limit and reaches a maximum at an intermediate frequency. The image impedance of the wsection, or mid shunt termination; passes through a re ciprocal set of valvesbeing infinite at the band limits and reaching al minimum at an intermediate frequency. The maximum impedance of the T section is equal `to the miniinum impedance of the vr sect-ion and occurs atthe same frequency, namely, the 'geometric mean of the band limiting frequencies.

The' impedance at this mean frequency Losanna In practically all cases it is found that the value of the nominal impedance is expressed by a very simple formula involving the' constants of the elements only. For the special cases represented by lowpass filters and high-pass filters, one band limit is at zero frequency or at infinity, and the geometric mean frequency corresponding to the nominal impedance is also at zero or infinity.

The impedance of a. power line, or other transmission line, is generally fairly constant and of a comparatively low value. 0n the other` hand, the impedance of a modulating or amplifying device is generally much greater,- so that in .superimposngmodulated carrier currents upon a power line a stepping down of the voltage corresponding to the ratio of the' impedances' is necessary.

1f the impedance matches ,the terminal impedances,

it is necessary to proportion the filterto the power lineJA impedance, since' it would not be practicable to interpose a transformer between the filter/ and the-power line. Matching the impedance of the filter to that of the translating apparatus would'then be accomplished by insertinga transformer of appropriate transformation ratio.

ln accordance with the invention, however, the filter is designed to have a nominal impedance intermediate in value between the two terminal impedances, preferably several times greater than the 'power line impedance. ln relation to the image impedances indicated in Fig. v 3, and Fig. 4, the power line impedance would be represented by the linelDD1 in Fig. 3 and the impedance of the translatingapparatus by GGl iu Fig. 4.

Considering the curves of Figures 3 and 4 it will be seen that the terminal impedances, i. e., of the line and translating apparatus, are matched by the filter at points e and which arefairly close to the band limits and that there is a considerable dis-4 lband, dotted curve 14 represents the overall transmission characteristic Aof the systeni that is obtained when the filter is .des signed to match the terminal impedances at filter is designed sothat its nominal the mean frequency in the band, and curve 15 the corresponding 'characteristic obtained by matching the unequal terminal impedances to the filter impedances at frequencies close to the band limits.

The quantities that are compared to give the transmission. characteristics 14;- and 15 are'the actual output currents Ib as given by equation 5 and the reference Value given by I L .l bmfJzzZb which was discussed in connection with equation 5. To permit of comparison with the iilter transfer factor the transmission ratio is defined `as the natural logarithm of the ratio lCharacteristic curve 14 shows most efficient transmission at the middleof the band accompanied by a marked rounding-off near the band limits due. to terminal reflection losses. Curvel show that the reflection losses have been greatly reduced near the band limits andA increased at the "middle of the band. Curve 15 also shows that the steepness of has been greatly increased.

The increased efficiency of near the band limits permits the two carrier Waves of the system of Fig. 1 to be more freely transmitted, which is .a desirable condition in a two-way signalling system of the type described.. Y

The refiection loss that is thrown to the mid'dleof the` band has -a maximum value equal to that due to the disparity 'of the two terminal impedances and itis found that impedances differing in the ratio of 10 to 1 may be linked together without increasing lthis loss to a degree that would noticeably `ofthe' high voltage condensers' 12. As the filter impedance is increased the impedance of'each branch is proportionately increased, this being achieved by making -the inductancesproportionately greater and the capacities smaller in an. inverse proportion.

This may be illustrated by citing the values computed for a typical system.

transmission .y

Vthe characteristic at the edges of the band Let it be assumed that the power line impedance is 400 ohms and thatthe impedance of the translating apparatus is 4000 ohms, these figures 'being close to. values found in actual practice.

If the coupling filter is designed to have a nominal impedance of 400 ohms and-to have band limits at 70 K. C. and 110 K. C. it is found that the capacity of condensers 12 should be equal to .004 microfarads. By increasing the filter impedance to 1600 ohms the coupling condenser capacities are reduced to one quarter, or .001 microfarads.

to isolate line voltages as high as 120 kilovolts in which case they become Very large and costly even for small capacities.

' Some idea of the difiiculties involved in the construction of these high voltage condensers may be gained from the fact that condensers of .003 microfarad capacity built to operate on a-120 kilo-volt line weighed 8000 pounds and occupied a space of 5 feet'in diameterl and l2 feet high. The economic advantage of using the smallest possible capacities is apparent.

The

has related more specifically to the case in which a filter is utilized to interconnect two 4unequal impedances, the filter being designed to have a nominal impedance of intermediate value. -The arrangement is, however, ca able ofvariation without substantially modifying` 'the results obtained.

For example the two terminal impedancesmay be equal and of low value to begin with.

-In this case. the filter may be terminated in In another application the filter may bel i matched at the mean frequency to the greater of two unequal impedances and at frequencies close to the band limits to the smaller impedance. In this way the refiection losses instead of occurring all at the band edges or all at the centre of the band are divided in a fairly uniform manner throughout the band.

Other possible variations naturally suggest themselves in which the basic principle of the invention is retained, namely, that the transmission loss Within the frequency band to be transmitted is removed at least in part from the` edges of the band to the centre by designing the filter so that its nominal image impedance is greatly -difl'erent Ifrom one or bothof the terminal impedances.

What is claimed is:

l. A frequency selective wave transmission system comprising a Wave lter having two pairs of terminals and having transmission devices representing terminal impedances connected to said pairs of terminals, said filter being so'proportioned that the image impedance at-one pair at least of its foregoing v description and discussion l terminals is greatly different from the terminal impedance connected' thereto throughout of said llimit. The coupling condensers may be required 1 2. A frequency selective wave transmission system comprising a broad band wave filter having two pairs of terminals and having transmission devices representing terminal impedances connected to said pairs of terminals, said Alter being so proportioned that its image impedances at said pairs of terminals are greatly different from the impedances of said terminal apparatus throughout the major portion of the filter transmission band and are equal to the connected terminal impedances at frequencies close to the limits of the transmission band, whereby refiection loss iseliminated in the neighborhood of the band limits.

3. A frequency selective wave transmission system comprising a broad band wave filter, two pairs of terminals therefor, and transmission devices representing terminal impedances, connected to said terminalsfone of said terminal impedances being much greater than the other, and. said filter being so proportioned that its nominal image impedance has a value intermediate between the terminal impedances and lthat at frequencies close to the band limits the image impedances are equal to ythe terminal impedances, whereby refiection loss is eliminated at frequencies close to the band limits.

4. `In a frequency selective transmission system, a broad band wave filter, and transmission devices representing terminal impedances connected thereto, one of said terminal impedances being much greater than the other and said filter having a nominal `impedance at the mid-band frequency intermediate between 'said terminal impedances and having series and shunt terminations at nol the ends adjacent to the low and the hi h terminal impedances respectively where y ductors', the image impedances-of said wave filter having a nominal value much greater thanttheimpedance of said conductors and having values close to the band limits which match the impedances of said conductors` and said translating apparatus, whereby there are provided two frequency ranges of minimum loss close to the band limits, in

las

which ranges separate .signal 'channels AAma),7 be'located.

6. In a power line-communication system," a transmission line ada. ted to transmithigh voltagev energy, signa "translating appa# ratus adapted for signalling by 'means of modulated carrier waves, and a broad band Wave'lter connected between said line and said apparatus, said filter being terminated adjacent to .the transmission line in. series branches.;y including condensers and having l an imagel impedance, at the line end which greatly -v exceeds the line impedance at the middle of the band and is equal -thereto atl frequencies close to the band 1imits,.where f by efficient coupling is secured at .selected frequencies bymeans of vlow capacity conlimits vand dier dansers vin inter. r

7.- As a'coupling element'between sectionsv` of a wave transmission having `una band ,wave filter equal ixnpedances, a bro #haterminatingfbfanchSf-@f f roportionedtolhave imagel mpedances at its terminals 'which match the impedances lto .f becoupled atjfrequencigs close to vthepbanol eatly'v therefrom at i'ntermediate frequen" es withinv the band,.where eicient transmission is secured atA select- TmofrHr msnm. A 

